In recent years, wireless communication devices which perform wireless communications by a high-frequency signal, such as microwave signals, are being used in many locations, such a cellular phone base stations. In order to receive electromagnetic waves of a band centered on a desired resonance frequency, these wireless communication devices incorporate a high-frequency filter composed by coupling a plurality of resonators which resonate at the resonance frequency in series. If the high-frequency filter is composed of N stages, then typically, N resonators which each resonate in one mode are coupled in succession. On the other hand, it has also been proposed to use a multi-mode resonator in which a plurality of resonant modes are degenerated by setting substantially the same resonance frequency, in such a manner that a high-frequency filter can be composed by a smaller number of resonators than N, and can be made compact in size.
For example, Patent Document 1 describes a multi-mode resonator in which four resonance modes degenerated. FIGS. 18A, 18B, 19A, 19B, 20A, 20B, 21A, 21B show the composition and electric field distribution of a multi-mode resonator 101 that is substantially the same as that described in Patent Document 1. In FIGS. 18A, 18B, 19A, 19B, 20A, 20B, 21A, 21B, the portion of the exterior conductor 102 in front of the internal members is depicted transparently so as to show the internal members. Furthermore, the electric field is indicated by the solid arrow lines. The magnetic field is not illustrated, but is distributed so as to circle around the electric field.
In this multi-mode resonator 101, a dielectric core 105 having a high permittivity of a special shape (an approximately octagonal columnar shape) is provided inside a box-shaped exterior conductor (cavity) 102. An electromagnetic field having a shorter wavelength due to the wavelength shortening effect of the high permittivity is localized and resonates in the dielectric core 105. Here, three resonance modes, namely, two TM modes (TM01δx mode and TM01δy mode) and one TE mode (TE01δ mode) degenerate. The two TM modes are modes where the electric field runs respectively in the X axis direction and the Y axis direction, and the electromagnetic field resonates with a half wavelength, as shown in FIGS. 18A, 18B, 19A, 19B. As shown in FIGS. 20A and 20B, the TE mode is a mode where the electric field runs in a circumferential direction about the central axis of the dielectric core 105, and the electromagnetic field resonates with a wavelength of one.
Moreover, in order to increment the number of resonance modes by one, a columnar central conductor 103 of which one end is shorted to an exterior conductor 102 is added so as to be inserted through a central hole formed in the central axis of the dielectric core 105. By means of this composition, a resonance mode of a so-called semi-coaxial resonator is obtained. As shown in FIGS. 21A and 21B, the resonance mode of a semi-coaxial resonator is a TEM mode in which the electric field runs radially from the central conductor 103 to the exterior conductor 102, and the electromagnetic field is distributed along the central conductor 103 and resonates with a quarter wavelength. The resonance frequency in this TEM mode is aligned with the resonance frequency of the two TM modes and the TE mode described above, by altering the outer diameter of the central conductor 103, for example, in such a manner that the distance of the gap from the central conductor 103 to the exterior conductor 102 is shorter on the side of the shorting terminal 103a compared to the open end 103b of the central conductor 103.